Blow-molding is a process for molding a preform part into a desired product. The preform is in the general shape of a tube with an opening at one end for the introduction of pressurized gas, typically air; however, other gases may be used. One specific type of blow-molding is stretch blow-molding (SBM). In typical SBM applications, both low and high-pressure gas is used to expand the preform into a mold cavity. The mold cavity comprises the outer shape of the desired product. SBM can be used in a wide variety of applications; however, one of the most widely used applications is in the production of Polyethylene terephthalate (PET) products, such as drinking bottles.
Typically, the SBM process uses a low-pressure fluid supply along with a stretch rod that is inserted into the preform to stretch the preform in a longitudinal direction and radially outward and then uses a high-pressure fluid supply to expand the preform into the mold cavity. The low-pressure fluid supply along with the stretch rod is typically referred to as a pre-blowing phase of the molding cycle. The high-pressure fluid supply that expands the preform into the mold cavity is typically referred to as the blowing phase of the molding cycle. The low-pressure and high-pressure fluid supplies can be controlled using blow-mold valves. The resulting product is generally hollow with an exterior shape conforming to the shape of the mold cavity. The gas in the preform is then exhausted through one or more exhaust valves. This process is repeated during each blow-molding cycle.
One of the more critical steps in the molding process occurs during the pre-blowing phase. During this phase, a pressure up to approximately 12 bar (174 psi) is provided to the preform while a stretch rod simultaneously extends the preform in a longitudinal direction. During the pre-blowing phase, there is an attempt to substantially uniformly distribute the material of the preform along the longitudinal length prior to expansion of the preform against the mold cavity. If the perform experiences a sudden jump in pressure during the pre-blowing phase, uniform distribution of the perform material may not be possible. In order to uniformly distribute the perform material, the pressure inside the preform must be carefully controlled.
Simply applying a pressurized gas source to a perform through a fixed orifice, aperture, or restrictor generates a steep, abrupt increase in pressure. Example pressure profiles 102, 104, and 106 are depicted in FIG. 1. The x-axis in FIG. 1 represents time, and the y-axis represents pressure. Pressure profiles 102, 104, and 106 depict the increase in pressure inside a perform using a 4 bar, 7 bar, and 10 bar respective pressurized gas source. In each instance, pressure profiles 102, 104, and 106 approach equilibrium rapidly with the pressurized gas source in roughly the same amount of time.
It is possible to control the pressure profile inside a perform by including a throttle valve between the source of pressurized gas and the preform. For example, FIG. 2 depicts a series of pressure profiles generated during a blow-molding process in accordance with an embodiment. The x-axis in FIG. 2 represents time, and the y-axis represents pressure. Pressure profiles 202, 204, 206, 208, 210, and 212 each increase at different rates to pressure level 214, the pressure profiles being controlled via a variable throttle.
One design for a variable throttle includes an outlet orifice and a piston that may be moved to block a portion of the surface area of the outlet orifice to increase or reducing the fluid flow through the orifice. By controlling the fluid flow through the outlet orifice of the throttle, it is possible to more carefully control a pressure profile inside a preform.
In blow-molding manufacturing, it is desirable to use the same equipment with minimal adjustments to fabricate multiple bottle sizes. In operating a throttle to control a pressure profile inside a preform, a piston may be stepped or moved through a stroke profile that includes a start position, a distance to actuate, and a rate to move. If the piston can be stepped through the same stroke profile for a wide variety of products, the need to reconfigure the piston stroke during the equipment setup can be minimized. For example, a piston stroke profile may include ten rotary turns of a piston to move the piston from a nearly closed position wherein the piston covers 100% of the surface area of an orifice to a position where the rotary piston is open and covers 0% of the surface area of the orifice.
Typical blow-molding bottle sizes may vary in volume from 0.2 L to 3.0 L, providing a volume range factor of 3.0/0.2=15. Pre-blowing pressures typically range between 2 bar and 12 bar, however, providing a pressure range factor of 12/2=6. Because the volume range factor (15) and pressure range factor (6) are different, the resolution of a throttle with a single output orifice cannot provide adequate flow resolution to match the extreme range of bottle volumes from 0.2 L to 3.0 L. In other words, a throttle outlet orifice that is ideal for a 3.0 L bottle size, providing adequate flow resolution to provide a desired pressure profile when a piston is moved across the surface area of the outlet orifice, will not provide an adequate resolution for a bottle side of 0.2 L. In order to generate any of the pressure profiles depicted in FIG. 2 for a variety of bottle sizes, a manufacturer will need adequate pressure resolution for each bottle size being fabricated.
FIG. 3 depicts pressure profiles in accordance with an embodiment. FIG. 3 illustrates the difference in pressure resolution that occurs when blowing extreme bottle sizes using the same throttle orifice surface area and blow-molding valve. In FIG. 3, the x-axis represents time when a perform is filling to a pre-blowing pressure level and the y-axis represents pressure inside a preform. Curves 302 and 306 represent the pressure inside a perform for a 0.2 L bottle, and curves 304 and 308 represent the pressure inside a perform for a 1.5 L bottle. In pressure profiles 302 and 304, the piston is positioned so that the outlet orifice of the throttle is 100% open. In pressure profiles 306 and 308, the piston is positioned so that the outlet orifice of the throttle covers 10% of the surface area of the orifice. As may be seen, the difference in time between when the 0.2 L bottle reaches the pre-blowing pressure level with each of the two piston positions is represented by the double-sided arrow 310. The difference in time between when the 1.5 L bottle reaches the pre-blowing pressure level with each of the two piston positions is represented by the double-sided arrow 312. Arrow 310 is shorter than arrow 312, indicating that for the given outlet orifice, the 1.5 L bottle size has more pressure resolution than the 0.2 L bottle size. In other words, the pressure resolution of the blow-molding valve and throttle is dependent on the bottle size being blown.
FIG. 4 depicts further pressure profiles in accordance with an embodiment. FIG. 4 is similar to FIG. 3, except that pressure profile 306, the pressure profile for a 1.5 L bottle with 10% of the surface area of the outlet orifice of the throttle open, is less steep than in FIG. 3. In FIG. 4, arrows 310 and 312 are the same length, meaning that the 0.2 L bottle size has the same pressure resolution as the 1.5 L bottle size.
There is a need in the art for a throttle that is easy to configure and operate to provide resolution under a variety of conditions to finely control pressure. The present embodiments described below overcome these and other problems and an advance in the art is achieved.